Golf ball

ABSTRACT

The invention provides a golf ball which includes a core and a cover encasing the core, wherein the cover has a thickness of at least 1.3 mm but not more than 1.8 mm and a material hardness, expressed as the Shore D hardness, of not more than 58, the core has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of at least 4.1 mm, the ball has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of at least 3.8 mm, and the ratio of the core deflection to the ball deflection (core/ball) is at least 0.8 but not more than 1.2. The ball of the invention can be sufficiently deformed on shots with a driver, even when played by a low head speed golfer, allowing a good ball rebound to be obtained and thus resulting in an increased distance. The ball also has a good feel when played and a satisfactory durability.

BACKGROUND OF THE INVENTION

The present invention relates to a golf ball for women golfers andjunior golfers which has a soft feel on impact and is able to achieve anexcellent flight performance even when struck at a low head speed.

To address the needs of professional golfers and skilled amateurs, manygolf balls have hitherto been developed which, when hit at a high headspeed, have an excellent flight performance and spin properties and alsoprovide a good feel on impact. However, because such balls are generallydesigned to achieve an optimal deformation when struck at a high headspeed, a sufficient deformation often cannot be imparted to the ballwhen played by women golfers and junior golfers having a low head speed.Hence, low head-speed golfers have been unable to make the most of theperformance inherent to such balls, failing to achieve a sufficientdistance, and have also experienced a poor feel on impact.

Consequently, developing a golf ball which allows even a low head-speedgolfer to impart sufficient deformation to the ball and which enables anexcellent flight performance and a good feel on impact to be achieved isimportant for expanding the golfer base.

Prior-art relating to this invention includes JP 2822926 (U.S. Pat. No.5,695,413), U.S. Pat. No. 6,142,886, JP 2576787 (U.S. Pat. No.5,452,898), U.S. Pat. No. 5,743,817 and U.S. Pat. No. 5,813,924.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a golfball which has a soft feel and enables an excellent flight performanceto be obtained even when hit by a low head-speed golfer.

As a result of extensive investigations, the inventor has discoveredthat, by forming a soft, thin cover around a soft core which has beenset to a large deflection, it is possible to confer the ball with a goodfeel and a satisfactory durability while at the same time enablingsufficient deformation to be imparted to the ball on shots with adriver, even when struck under low head speed conditions. This allows agood ball rebound to be obtained and also suppresses an excessive spinrate, resulting in an increased distance.

Accordingly, the invention provides the following golf balls.

[1] A golf ball comprising a core and a cover encasing the core, whereinthe cover has a thickness of at least 1.3 mm but not more than 1.8 mmand a material hardness, expressed as the Shore D hardness, of not morethan 58, the core has a deflection when compressed under a final load of1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of atleast 4.1 mm, the ball has a deflection when compressed under a finalload of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) ofat least 3.8 mm, and the ratio of the core deflection to the balldeflection (core/ball) is at least 0.8 but not more than 1.2.[2] The golf ball of claim 1, wherein the cover is formed of a resincomposition which is composed primarily of an ionomer and which has amelt flow rate of at least 4.[3] The golf ball of claim 2, wherein an ethylene-methacrylicacid-methacrylic acid ester copolymer resin accounts for between 10 and80 wt % of the polymer components included in the ionomer composition.[4] The golf ball of claim 1, wherein the cover is formed of a resincomposition which is composed primarily of polyurethane and which has amelt flow rate of at least 16.

BRIEF DESCRIPTION OF THE DIAGRAM

FIG. 1 is a schematic cross-sectional view of a golf ball according toan embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The golf ball is described more fully below.

Referring to FIG. 1, the golf ball of the invention has a constructionwhich is exemplified by a two-piece solid golf ball G composed of a core1 and a cover 2 encasing the core. The cover 2 typically has a surfaceon which numerous dimples D are formed. Here, the core 1 and the cover 2may each be composed of a single layer, although it is also possible forthe core to be given a multilayer construction of from 2 to 6 layers,and for the cover to be composed of a plurality of two or more layers.

The core may be obtained by vulcanizing a rubber composition composedprimarily of a rubber material. Specifically, use may be made of arubber composition containing, for example, a base rubber, acrosslinking initiator, a co-crosslinking agent, an antioxidant and afiller.

The base rubber of the rubber composition is not subject to anyparticular limitation, although it is preferable to use polybutadiene.Preferred use may be made of cis-1,4-polybutadiene having a cisstructure of at least 40% as the polybutadiene. In addition, naturalrubber, polyisoprene rubber, styrene butadiene rubber and the like maybe suitably included, where desired, in this base rubber. The rebound ofthe golf ball can be enhanced by increasing the amount of rubbercomponents.

In the invention, preferred use may be made of an organic peroxide asthe crosslinking initiator. Preferred examples of organic peroxides thatmay be used include dicumyl peroxide and1,1-bis(tert-butylperoxy)cyclohexane. A commercial product may be usedas the organic peroxide. Illustrative examples include Perhexa 3M,Perhexa C40, Percumyl D (NOF Corporation), Luperco 231XL and Luperco101XL (Atochem Co.). These may be used singly or as mixtures of two ormore thereof.

The amount of crosslinking initiator included, although not subject toany particular limitation, is preferably at least 0.2 part by weight,more preferably at least 0.4 part by weight, and even more preferably atleast 0.6 part by weight, per 100 parts by weight of the base rubber.The upper limit in the amount included, although not subject to anyparticular limitation, may be set to preferably not more than 2.0 partsby weight, more preferably not more than 1.5 parts by weight, even morepreferably not more than 1.2 parts by weight, and most preferably notmore than 0.9 part by weight. If the amount included is too high, thismay cause scorching during rubber compounding and hot molding, in whichcase a golf ball core and golf ball of sufficient durability will not beobtained. If too little is included, the workability when hot moldingthe rubber composition may decrease, as a result of which a hardnesssufficient for the golf ball core may not be achieved. Although notsubject to any particular limitation, in this invention, sulfur may alsobe included in the above rubber composition. The sulfur is exemplifiedby powdered sulfur, illustrative examples of which include the productavailable under the trade name Sulfur Z (dispersible sulfur) fromTsurumi Chemical Industry Co., Ltd. The amount of sulfur included per100 parts by weight of the above base rubber is typically from 0.01 to0.5 part by weight, preferably from 0.01 to 0.4 part by weight, and morepreferably from 0.01 to 0.1 part by weight. If the amount of sulfurincluded is too low, the hardness distribution of the solid core may belimited in the degree to which it can be increased, as a result of whichthe rebound resilience may become lower, resulting in a shorterdistance. On the other hand, if the amount of sulfur included is toohigh, undesirable effects such as explosion of the rubber compositionmay arise during hot molding.

α,β-Unsaturated carboxylic acids such as zinc methacrylate and zincacrylate may be included as the co-crosslinking agent in the presentinvention. The use of zinc acrylate is especially preferred. The amountof co-crosslinking agent included is not subject to any particularlimitation, but may be set to at least 10 parts by weight, andpreferably at least 15 parts by weight, per 100 parts by weight of thebase rubber. The upper limit in the amount of co-crosslinking agent isnot subject to any particular limitation, although it is recommendedthat it be set to not more than 50 parts by weight, and preferably notmore than 39 parts by weight. If too much co-crosslinking agent isincluded, the core will become too hard, as a result of which the balltoo will be hard. For this reason, low head speed golfers may be unableto impart sufficient deformation to the ball at the time of impact, andso the ball may not achieve a good distance. On the other hand, if toolittle co-crosslinking agent is included, sufficient durability may notbe obtained.

In the invention, an antioxidant may be included in the rubbercomposition. For example, a commercial product such as Nocrac NS-6,Nocrac NS-30 or Nocrac SP-N (all available from Ouchi Shinko ChemicalIndustry Co., Ltd.) may be used. These may be used singly or ascombinations or two or more thereof.

The amount of antioxidant included per 100 parts by weight of the baserubber, although not subject to any particular limitation, is preferablyat least 0.02 part by weight, and more preferably at least 0.05 part byweight. The upper limit is preferably not more than 1 part by weight,more preferably not more than 0.8 part by weight, and even morepreferably not more than 0.6 part by weight. If the amount included istoo high, the golf ball may not be able to achieve a sufficient initialvelocity. On the other hand, if the amount is too low, this may causescorching during rubber compounding and hot molding, in which case agolf ball core and golf ball having sufficient durability may not beattainable.

The filler is not subject to any particular limitation. Illustrativeexamples of fillers that may be included are zinc oxide, barium sulfate,titanium dioxide and calcium carbonate. The amount in which these areincluded is not subject to any particular limitation, although theamount may be set to at least 3 parts by weight, and preferably at least5 parts by weight, per 100 parts by weight of the base rubber. The upperlimit in the amount of filler included is not subject to any particularlimitation, although it is recommended that this be set to not more than60 parts by weight, and preferably not more than 50 parts by weight.

The core can be produced by using a known method to vulcanize the rubbercomposition containing the various above ingredients. For example, thevarious ingredients may be kneaded together using a mixing apparatussuch as a Banbury mixer or a roll mill so as give a rubber composition,and the composition may then be compression-molded or injection-moldedusing a core mold to as to give a spherical molding. The molding is thensuitably heated under conditions sufficient for the crosslinkinginitiator and co-crosslinking agent to act and is thereby cured, givinga core having a given hardness. In cases where, for example, dicumylperoxide is used as the crosslinking initiator and zinc acrylate is usedas the co-crosslinking agent, the heating conditions for the molding maybe set to generally about 130 to 170° C., and especially 150 to 160° C.,and to 10 to 40 minutes, and especially 12 to 20 minutes.

The core diameter, although not subject to any particular limitation, ispreferably at least 24 mm, more preferably at least 26 mm, and even morepreferably at least 28 mm. The upper limit is preferably not more than42 mm, more preferably not more than 41.5 mm, and even more preferablynot more than 41 mm. At a core diameter outside of this range, asufficient rebound performance may not be achieved, excess spin mayarise, or a good feel may not be obtained.

It is critical for the core to have a deflection, when compressed undera final load of 1,275 N (130 kgf) from an initial load state of 98 N (10kgf), of at least 4.1 mm, preferably at least 4.2 mm, and morepreferably at least 4.3 mm. The upper limit in the deflection, althoughnot subject to any particular limitation, is preferably not more than8.0 mm, more preferably not more than 7.0 mm, even more preferably notmore than 6.0 mm, and most preferably not more than 5.5 mm. If the coreis softer than the above value, the core rebound may be poor. On theother hand, if the core is harder than the above value, the ball mayhave a poor feel.

In the invention, the cover formed over the core may be made of a knownmaterial. Although not subject to any particular limitation, exemplarycover materials include thermoplastic resins such as ionomers, andvarious types of thermoplastic elastomers. Specific examples ofthermoplastic elastomers include polyester-type thermoplasticelastomers, polyamide-type thermoplastic elastomers, polyurethane-typethermoplastic elastomers, olefin-type thermoplastic elastomers, andstyrene-type thermoplastic elastomers.

In the invention, such a cover material is not subject to any particularlimitation, although preferred use may be made of the following resincompositions composed primarily of an ionomer or a polyurethane. Each ofthese materials is described in turn below.

First, the resin composition composed primarily of an ionomer is, inthis invention, a resin composition composed primarily of a metal saltof an olefin-unsaturated carboxylic acid-unsaturated carboxylic acidester random copolymer and/or a metal salt of an olefin-unsaturatedcarboxylic acid random copolymer.

The olefin is exemplified by olefins in which the number of carbons isat least 2, but not more than 8, and preferably not more than 6.Illustrative examples of such olefins include ethylene, propylene,butene, pentene, hexene, heptene and octene. Ethylene is especiallypreferred.

Illustrative examples of the unsaturated carboxylic acid include acrylicacid, methacrylic acid, maleic acid and fumaric acid. In the invention,of these, acrylic acid and methacrylic acid are preferred, andmethacrylic acid is especially preferred.

The unsaturated carboxylic acid ester included is preferably a loweralkyl ester of the above-mentioned unsaturated carboxylic acid.Illustrative examples include methyl methacrylate, ethyl methacrylate,propyl methacrylate, butyl methacrylate, methyl acrylate, ethylacrylate, propyl acrylate and butyl acrylate. Butyl acrylate (n-butylacrylate, i-butyl acrylate) is especially preferred.

The random copolymer may be obtained by random copolymerization of theabove ingredients in accordance with a known method. Here, the contentof unsaturated carboxylic acid (acid content) included in the randomcopolymer, although not subject to any particular limitation, isgenerally at least 2 wt %, preferably at least 6 wt %, and even morepreferably at least 8 wt %. It is recommended that the upper limit inthe unsaturated carboxylic acid content (acid content), although notsubject to any particular limitation, be generally not more than 25 wt%, preferably not more than 20 wt %, and more preferably not more than15 wt %. At a low acid content, the rebound may decrease, whereas at ahigh acid content, the processability of the material may decrease.

The acid groups in the above random copolymers are partially orcompletely neutralized with metal ions. In this case, although thedegree of neutralization is not subject to any particular limitation, itis preferable for at least 20 mol % of the acid groups to beneutralized; this proportion may be set to more preferably at least 30mol %, and even more preferably at least 40 mol %. The upper limit alsois not subject to any particular limitation, although it may be set tonot more than 100 mol %, preferably not more than 90 mol %, and evenmore preferably not more than 80 mol %. At a degree of neutralizationlower than 20 mol %, the rebound may become too low. Here, the metalions which neutralize the acid groups are exemplified by Na⁺, K⁺, Li⁺,Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. In the invention, of these,Na⁺, Li⁺, Zn⁺⁺, Mg⁺⁺ and Ca⁺⁺ are especially preferred.

The content of the above metal salt of a random copolymer (ionomerresin) is not subject to any particular limitation, although it ispreferable for between 100 and 20 wt %, based on the overall resincomposition, to be included. In this case, the lower limit is morepreferably at least 40 wt %, and even more preferably at least 50 wt %.The upper limit is more preferably not more than 90 wt %, and even morepreferably not more than 85 wt %.

Although not subject to any particular limitation, from the standpointof providing good flow properties and processability during injectionmolding, it is preferable for the proportion of ethylene-methacrylicacid-methacrylic acid ester copolymer included in the ionomercomposition as a polymer component other than the ionomer to be from 10to 80 wt %. An illustrative example is that available under the tradename Nucrel AN4319 (DuPont-Mitsui Polychemicals Co., Ltd.). The lowerlimit value of this ratio is preferably at least 15 wt %, and morepreferably at least 20 wt %. The upper limit value is preferably notmore than 70 wt %, more preferably not more than 60 wt %, and even morepreferably not more than 50 wt %. If the flow properties are too low,forming a thin cover over the core by an injection-molding process maybe difficult, as a result of which production may be inefficient.

In the invention, a known product may be used as the above ionomer.Illustrative examples include the products available under the tradenames Himilan 1557 and Himilan 1601 (DuPont-Mitsui Polychemicals Co.,Ltd.), the products available under the trade names HPF1000 and HPF2000(available from E.I. DuPont de Nemours and Co.), and the resincompositions mentioned in U.S. patent application Ser. No. 12/340,790(or U.S. patent application Ser. No. 12/706,175).

These may be used singly or as mixtures of two or more thereof.

Various additives may be optionally included in the above resincomposition. For example, additives such as pigments, dispersants,antioxidants, UV absorbers and optical stabilizers may be suitablyincluded.

The above resin composition composed primarily of an ionomer has a meltflow rate (MFR) which is not subject to any particular limitation.However, to provide good flow properties and processability duringinjection molding, the melt flow rate, as measured at a test temperatureof 190° C. and under a test load of 21.18 N (2.16 kgf) in generalaccordance with JIS K-7210, is preferably at least 4 g/10 min, morepreferably at least 5 g/10 min, and even more preferably at least 6 g/10min. The upper limit also is not subject to any particular limitation,but is recommended to be not more than 14 g/10 min, and more preferablynot more than 13 g/10 min. If the flow properties are too low, forming athin cover over the core by an injection-molding process may bedifficult, as a result of which production may be inefficient.

Next, in cases where the cover is formed using a resin compositioncomposed primarily of a polyurethane, a thermoplastic polyurethaneelastomer or a thermoset polyurethane resin may be used in theinvention. The use of a thermoplastic polyurethane elastomer isespecially preferred.

The thermoplastic polyurethane elastomer has a structure composed ofsoft segments made of a polymeric polyol (polymeric glycol) and hardsegments made of a chain extender and a diisocyanate. Here, thepolymeric polyol serving as a starting material may be any which hashitherto been used in the art relating to thermoplastic polyurethanematerials, and is not subject to any particular limitation. Exemplarypolymeric polyols include polyester polyols and polyether polyols.Polyether polyols are more preferable than polyester polyols becausethermoplastic polyurethane materials having a high rebound resilienceand excellent low-temperature properties can be synthesized.Illustrative examples of polyether polyols include polytetramethyleneglycol and polypropylene glycol. Polytetramethylene glycol is especiallypreferred from the standpoint of the rebound resilience and thelow-temperature properties. The polymeric polyol has an averagemolecular weight of preferably from 1,000 to 5,000. To synthesize athermoplastic polyurethane material having a high rebound resilience, anaverage molecular weight of from 2,000 to 4,000 is especially preferred.

The chain extender employed is preferably one which has hitherto beenused in the art relating to thermoplastic polyurethane materials.Illustrative examples include, but are not limited to, 1,4-butyleneglycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and2,2-dimethyl-1,3-propanediol. These chain extenders have an averagemolecular weight of preferably from 20 to 15,000.

The diisocyanate employed is preferably one which has hitherto been usedin the art relating to thermoplastic polyurethane materials.Illustrative examples include, but are not limited to, aromaticdiisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluenediisocyanate and 2,6-toluene diisocyanate, and aliphatic diisocyanatessuch as hexamethylene diisocyanate. Depending on the type of isocyanate,control of the crosslinking reaction during injection molding may bedifficult. In this invention, the use of 4,4′-diphenylmethanediisocyanate, which is an aromatic diisocyanate, is most preferred.

A commercial product may be advantageously used as the thermoplasticpolyurethane material composed of the above materials. Illustrativeexamples include those available under the trade names Pandex T8180,Pandex T8195, Pandex T8290, Pandex T8295 and Pandex T8260 (all availablefrom DIC Bayer Polymer, Ltd.), and those available under the trade namesResamine 2593 and Resamine 2597 (available from Dainichiseika Color &Chemicals Mfg. Co., Ltd.).

Various additives may be optionally included in the above thermoplasticpolyurethane material. For example, additives such as pigments,dispersants, antioxidants, UV absorbers and optical stabilizers may besuitably included.

In the invention, the resin composition composed primarily of apolyurethane has a melt flow rate which is not subject to any particularlimitation. However, to provide good flow properties and processabilityduring injection molding, the melt flow rate, as measured at a testtemperature of 210° C. and under a test load of 21.18 N (2.16 kgf) ingeneral accordance with JIS K-7210, is preferably at least 16 g/10 min,more preferably at least 17 g/10 min, and even more preferably at least18 g/10 min. The upper limit also is not subject to any particularlimitation, but is recommended to be not more than 40 g/10 min, and morepreferably not more than 35 g/10 min. If the flow properties are toolow, forming a thin cover over the core by an injection-molding processmay be difficult, as a result of which production may be inefficient.

It is critical for the cover thickness to be set to at least 1.3 mm. Theupper limit is not more than 1.8 mm, and preferably not more than 1.7mm. At a cover thickness outside of the above range, a good feel onimpact may not be obtained. Also, a sufficient durability may not beobtained.

The cover has a material hardness, expressed as the Shore D hardness,which must be not more than 58, and may be set to preferably not morethan 57, and more preferably not more than 56. The lower limit is notsubject to any particular limitation, but may be set to a JIS-A hardnessof preferably at least 30, more preferably at least 35, even morepreferably at least 40, yet more preferably at least 45, and mostpreferably a Shore D hardness of at least 50. If the material hardnessof the cover is too high, the cover will have difficulty following thecore deformation when the ball is struck, which may lead to a worseningin the durability of the ball to repeated impact. On the other hand, incases where the material hardness of the core is too low, the reboundwill often be inadequate. Here, with regard to the above materialhardnesses, the Shore D hardness is the hardness measured using a Type Ddurometer in general accordance with ASTM D2240 for a sheet of thecover-forming material that has been molded under pressure to athickness of about 2 mm; and the JIS-A hardness is the hardness measuredin general accordance with JIS K-6301 for a similar sheet.

The cover may be formed by a known method, such as injection-molding orcompression-molding. For example, when the cover is formed by injectionmolding, a solid core fabricated beforehand using the above-describedrubber composition may be set inside a core-forming mold, and theabove-described cover material may be injected into the mold by anordinary method. Alternatively, another method that may be used involvesusing the above-described cover material to pre-mold a pair ofhalf-cups, enclosing the above core in the half-cups, andcompression-molding at, for example, between 120 and 170° C. for aperiod of 1 to 5 minutes.

It is critical for the golf ball of the invention to have a deflection,when compressed under a final load of 1,275 N (130 kgf) from an initialload state of 98 N (10 kgf), of at least 3.8 mm. The deflection ispreferably at least 3.9 mm, and more preferably at least 4.0 mm. Theupper limit in this deflection, although not subject to any particularlimitation, is preferably not more than 8.0 mm, more preferably not morethan 7.0 mm, even more preferably not more than 6.0 mm, yet morepreferably not more than 5.0 mm, and most preferably not more than 4.7mm. If the ball is too soft, a suitable launch angle may not beobtained. On the other hand, if the ball is too hard, the ball may notfly well when struck by a low head-speed golfer, and the feel on impactmay be poor.

The ratio of the core deflection when compressed under a final load of1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) to theball deflection when compressed under a final load of 1,275 N (130 kgf)from an initial load state of 98 N (10 kgf) (core/ball) is at least 0.8,preferably at least 0.82, more preferably at least 0.84, and even morepreferably at least 0.86. The upper limit is not more than 1.2,preferably not more than 1.19, and more preferably not more than 1.18.If this ratio is outside the above range, the cover thickness andhardness will be inappropriate for the core deflection, and a good feelwill not be obtained at the time of impact.

In the golf ball of the invention, although not subject to anyparticular limitation, to further improve the aerodynamic properties andincrease the distance of the ball, as in conventional golf balls, aplurality of dimples may be formed on the surface of the cover. Byoptimizing the types and total number of such dimples, the trajectorycan be made more stable, enabling a ball having an excellent flightperformance to be obtained. In addition, to enhance the design anddurability of the golf ball, the cover surface may be painted, at whichtime various treatments such as surface preparation and stamping may beoptionally carried out.

The preferred form of the dimples in the golf ball of the invention isdescribed in detail below.

The number of dimples on the entire surface of the golf ball ispreferably at least about 200, more preferably at least about 220, evenmore preferably at least about 260, and yet more preferably at leastabout 300. The upper limit in the number of dimples is preferably notmore than about 450, more preferably not more than about 430, and evenmore preferably not more than about 410. By setting the number ofdimples within this range, the golf ball readily incurs lift, enablingthe distance to be increased, particularly on shots with a driver.

A plurality of types of dimples of differing diameter and/or depth maybe formed. The number of dimple types is preferably at least 4, morepreferably at least 5, and even more preferably at least 6. The upperlimit in the number of dimple types is preferably not more than about20, more preferably not more than about 15, and even more preferably notmore than about 12. By setting the number of dimple types in this range,the dimple surface coverage can be easily increased and the distancetraveled by the ball enhanced.

The shape of the dimples is preferably circular as seen from above. Theaverage diameter is preferably at least about 2.8 mm, more preferably atleast about 3.5 mm, and even more preferably at least about 3.8 mm. Theupper limit in the average diameter is preferably not more than about5.0 mm, more preferably not more than about 4.6 mm, and even morepreferably not more than about 4.3 mm. The average dimple depth, fromthe standpoint of obtaining a suitable trajectory, is preferably atleast about 0.120 mm, more preferably at least about 0.130 mm, and evenmore preferably at least about 0.140 mm. The upper limit in the averagedepth is preferably not more than about 0.185 mm, more preferably notmore than about 0.180 mm, and even more preferably not more than about0.174 mm.

As used herein, “average diameter” refers to the average of thediameters of all the dimples, and “average depth” refers to the averageof the depths of all the dimples. In most cases, the golf ball ispainted, with measurement of the dimple diameters and depths beingcarried out on the dimples after the coat of paint has been applied.

Measurement of a dimple diameter is carried out by measuring the spanacross the dimple between points where land areas (non-dimple formingportions) of the golf ball surface are tangent with the recessed surfaceof the dimple. Measurement of the dimple depth is carried out byconnecting points where the dimple is tangent with land areas so as totrace an imaginary circle in a flat plane, and measuring the verticaldistance from the center of the circle to the bottom of the dimple.

To fully manifest the aerodynamic properties, the dimple coverage or,specifically, the ratio (SR value) of the sum of the individual dimplesurface areas, each defined by the flat plane circumscribed by the edgeof the dimple, to the surface area of an imaginary sphere were the ballsurface assumed to have no dimples thereon, is preferably at least about60%, more preferably at least about 65%, and even more preferably atleast about 68%. In the invention, the upper limit in the dimplecoverage, although not subject to any particular limitation, ispreferably not more than about 90%, more preferably not more than about85%, and even more preferably not more than about 80%. From thestandpoint of optimizing the ball trajectory, the value V₀ obtained bydividing the spatial volume of the individual dimples below the flatplane circumscribed by the edge of each particular dimple by the volumeof the cylinder whose base is the flat plane and whose height is themaximum depth of the dimple from that base is preferably at least about0.35. The upper limit of V₀, although not subject to any particularlimitation, is preferably not more than about 0.80. In addition, the VRvalue, defined as the sum of the volumes of the dimples formed below theflat plane circumscribed by the edge of each dimple as a proportion ofthe volume of the imaginary sphere were the ball to have no dimplesthereon, is preferably at least about 0.6%, more preferably at leastabout 0.7%, and even more preferably at least about 0.8%. The upperlimit in the VR value is preferably not more than about 1.0%, and morepreferably not more than about 0.9%.

The golf ball of the invention may be manufactured so as to conform withthe Rules of Golf for competitive play, with the ball preferably beingset to a diameter of not less than 42.67 mm and a weight of not morethan 45.93 g.

As described above, the golf ball of the invention can be sufficientlydeformed on shots with a driver, even when played by a woman golfer orjunior golfer having a low head speed, allowing a good ball rebound tobe obtained, and thus enabling an increased distance to be achieved. Theball also has a good feel when played and a satisfactory durability.

EXAMPLES

Examples of the invention and Comparative Examples are given below byway of illustration, and not by way of limitation.

Examples 1 to 8, Comparative Examples 1 to 8

Rubber compositions were formulated as shown in Table 1 (Examples of theinvention) and Table 2 (Comparative Examples) below, then molded andvulcanized at 155° C. for 10 minutes to produce solid cores. Next,covers formulated as shown in Table 1 or Table 2 were injection-moldedover the cores obtained above, thereby giving two-piece solid golf ballsfor each Example of the invention and each Comparative Example.

TABLE 1 Example 1 2 3 4 5 6 7 8 Core Formulation Zinc oxide 17.2 18.519.7 22.4 22.8 11.2 12.2 21.6 (pbw) cis-1,4-Polybutadiene 100 100 100100 100 100 100 100 Dicumyl peroxide 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.61,1-Bis(tert-butylperoxy)- 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 cyclohexane2,2′-Methylenebis(4- 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1methyl-6-t-butylphenol) Zinc acrylate 26 23.5 21 20 19 25 23 21 Zincstearate 5 5 5 5 5 5 5 5 Titanium dioxide 2 2 2 2 2 2 2 2 Zinc salt of1.00 0.60 0.25 0.20 0.15 1.00 0.80 0.80 pentachlorothiophenol Diameter(mm) 40.0 40.0 40.0 39.3 39.3 39.3 39.3 39.3 Weight (g) 38.7 38.7 38.737.0 37.0 35.3 35.3 35.3 Deflection (mm) 4.3 4.4 4.5 4.6 4.7 4.4 4.5 5.2Cover Formulation Himilan 1557 27.5 32.5 37.5 37.5 42.5 27.5 (pbw)Himilan 1601 27.5 32.5 37.5 37.5 42.5 27.5 Nucrel AN4319 45 35 25 25 1545 Pandex T8260 55 70 Pandex T8195 45 30 Titanium dioxide 2.1 2.1 2.12.1 2.1 3.5 3.5 2.1 Polyethylene wax 1.5 1.5 MFR (g/10 min), 190° C. 9.68.1 6.7 6.7 5.2 9.6 MFR (g/10 min), 210° C. 18.7 18.2 Thickness (mm) 1.31.3 1.3 1.7 1.7 1.7 1.7 1.7 Material hardness (Shore D) 52 54 56 56 5852 54 52 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7Weight (g) 45.3 45.3 45.3 45.3 45.3 45.3 45.3 45.3 Deflection (mm) 4.04.0 4.0 4.0 4.0 4.0 4.0 4.6 Deflection ratio (core/ball) 1.08 1.09 1.111.14 1.18 1.10 1.11 1.13 Durability to cracking 31 25 15 25 0 57 48 0(difference in number of shots) Distance on shots with driver 0.4 0.20.9 0.2 0 0.4 0.5 0.2 (difference in m) Spin rate on shots with driver20 −10 −30 −25 −40 35 21 −10 (difference in rpm) Feel good good goodgood good good good good

TABLE 2 Comparative Example 1 2 3 4 5 6 7 8 Core Formulation Zinc oxide23.4 24.9 24.8 16.6 25.2 25.5 21.1 25.2 (pbw) cis-1,4-Polybutadiene 100100 100 100 100 100 100 100 Dicumyl peroxide 0.6 0.6 0.6 0.6 0.6 0.6 0.60.6 1,1-Bis(tert-butylperoxy)- 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6cyclohexane 2,2′-Methylenebis(4- 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1methyl-6-t-butylphenol) Zinc acrylate 25.5 22.5 23 22 22 21 18 22 Zincstearate 5 5 5 5 5 5 5 5 Titanium dioxide 2 2 2 2 2 2 2 2 Zinc salt of0.6 0.15 0.05 0.35 0.1 0.1 pentachlorothiophenol Diameter (mm) 38.6 38.638.6 41.1 38.6 38.6 40.0 38.6 Weight (g) 35.3 35.3 35.3 40.5 35.3 35.338.7 35.3 Deflection (mm) 4.5 4.7 4.5 4.4 4.6 4.8 4.7 4.7 CoverFormulation Himilan 1557 27.5 37.5 37.5 50 42.5 42.5 50 50 (pbw) Himilan1601 27.5 37.5 37.5 50 42.5 42.5 50 50 Nucrel AN4319 45 25 25 0 15 15Pandex T8260 Pandex T8195 Titanium dioxide 2.1 2.1 2.1 2.1 2.1 2.1 2.12.1 Polyethylene wax MFR (g/10 min), 190° C. 9.6 6.7 6.7 3.7 5.2 5.2 3.73.7 MFR (g/10 min), 210° C. Thickness (mm) 2.1 2.1 2.1 0.8 2.1 2.1 1.32.1 Material hardness (Shore D) 52 56 56 60 58 58 60 60 Ball Diameter(mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.3 45.3 45.345.3 45.3 45.3 45.3 45.3 Deflection (mm) 4.0 4.0 3.6 4.0 3.6 4.0 4.0 3.6Deflection ratio (core/ball) 1.13 1.16 1.25 1.10 1.28 1.20 1.18 1.31Durability to cracking 70 36 56 −23 25 5 −35 −15 (difference in numberof shots) Distance on shots with driver −2.8 −1.2 −3.2 1.0 −2.6 −0.6 1.1−3.0 (difference in m) Spin rate on shots with driver 80 0 70 −46 60 −20−55 50 (difference in rpm) Feel NG good NG NG NG NG NG NG

Details of the above core formulations are provided below.

-   Zinc oxide: Available from Sakai Chemical Co., Ltd.-   Polybutadiene: Available under the trade name “BR730” from JSR    Corporation-   Dicumyl peroxide: Available under the trade name “Percumyl D” from    NOF Corporation-   1 μl-Bis(tert-butylperoxy)cyclohexane, 40% dilution:    -   Available under the trade name “Perhexa C40” from NOF        Corporation-   2,2′-Methylenebis(4-methyl-6-t-butylphenol):    -   Available under the trade name “Nocrac NS-6” from Ouchi Shinko        Chemical Industry Co., Ltd.-   Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd.-   Zinc stearate: Available from NOF Corporation-   Titanium dioxide: Available under the trade name “Tipaque R550” from    Ishihara Sangyo Kaisha, Ltd.-   Zinc salt of pentachlorothiophenol:    -   Available from Tokyo Chemical Industry Co., Ltd.

Details of the above cover formulations are provided below.

-   Himilan 1557: A zinc ionomer of a binary copolymer, available from    DuPont-Mitsui Polychemicals Co., Ltd.-   Himilan 1601: A sodium ionomer of a binary copolymer, available from    DuPont-Mitsui Polychemicals Co., Ltd.-   Nucrel AN4319: A terpolymer available from DuPont-Mitsui    Polychemicals Co., Ltd.-   Titanium dioxide: Available under the trade name “Tipaque R550” from    Ishihara Sangyo Kaisha, Ltd.-   Pandex: An MDI-PTMG type thermoplastic polyurethane available from    DIC Bayer Polymer-   Polyethylene wax: Available under the trade name “Sanwax 161P” from    Sanyo Chemical Industries, Ltd.

The following physical properties of the resulting golf balls wereinvestigated. In addition, flight tests were carried out by the methoddescribed above, and the durability to cracking and feel on impact werealso evaluated. Those results are shown in Tables 1 and 2.

Material Hardness of Cover

The Shore D hardness, measured in general accordance with ASTM D-2240,of a test specimen obtained by melting the cover material (resincomposition) under applied heat and molding the material into a 2mm-thick sheet.

Core and Ball Deflections (mm)

The deflection (mm) when the test object was compressed under a finalload of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf)was measured.

Flight Performance

The distance traveled by the ball when struck at a head speed of 35 m/swith a driver mounted on a golf swing robot was measured. The club was aTourStage ViQ HT 460 driver (loft, 9.5°) manufactured by BridgestoneSports Co., Ltd. The differences in the measured values relative to areference value of “0” for Example 5 of the invention are shown in thetables.

Spin Rate of Ball (rpm)

The spin rate was measured under the same conditions as mentioned above.The differences in the measured values relative to a reference value of“0” for Comparative Example 2 are shown in the tables.

Durability to Cracking

The durability of the golf ball was evaluated using an ADC Ball CORDurability Tester produced by Automated Design Corporation (U.S.). Thistester functions so as to fire a golf ball pneumatically and cause it torepeatedly strike two metal plates arranged in parallel. The incidentvelocity against the metal plates was set to 43 m/s. For each ball, thenumber of shots that had been taken with the ball when the velocityratio (rebound velocity/incident velocity) fell below 97% was counted.The differences in the measured values relative to a reference value of“0” for Example 5 of the invention are shown in the tables.

Feel on Shots with a Driver

Five golfers having a head speed of about 35 m/s hit the test ball andrated the feel at that time according to the following criteria. Theclub used was a TourStage ViQ HT 460 driver (loft, 9.5°) manufactured byBridgestone Sports Co., Ltd.

Good: good feel

NG: poor feel

The balls in Comparative Examples 1, 2, 3, 5 and 6 all achieved a gooddurability to cracking, but were inferior in terms of distance. Theballs in Comparative Examples 4 and 7 achieved a good distance on shotswith a driver, but were inferior in terms of the durability to cracking.The ball in Comparative Example 8 had a poor distance on shots with adriver and a poor durability to cracking. The balls in ComparativeExample 1 and in Comparative Examples 3 to 8 all had a poor feel onimpact. By contrast with the balls in these comparative examples, theballs in Examples 1 to 8 of the invention all achieved a good distanceon shots with a driver and had a good durability to cracking, andmoreover had a good feel on impact.

1. A golf ball comprising a core and a cover encasing the core, whereinthe cover has a thickness of at least 1.3 mm but not more than 1.8 mmand a material hardness, expressed as the Shore D hardness, of not morethan 58, the core has a deflection when compressed under a final load of1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) of atleast 4.1 mm, the ball has a deflection when compressed under a finalload of 1,275 N (130 kgf) from an initial load state of 98 N (10 kgf) ofat least 3.8 mm, and the ratio of the core deflection to the balldeflection (core/ball) is at least 0.8 but not more than 1.2.
 2. Thegolf ball of claim 1, wherein the cover is formed of a resin compositionwhich is composed primarily of an ionomer and which has a melt flow rateof at least
 4. 3. The golf ball of claim 2, wherein anethylene-methacrylic acid-methacrylic acid ester copolymer resinaccounts for between 10 and 80 wt % of the polymer components includedin the ionomer composition.
 4. The golf ball of claim 1, wherein thecover is formed of a resin composition which is composed primarily ofpolyurethane and which has a melt flow rate of at least 16.